Such a circuit has particular advantages when applied in apparatus for controlling the power supply to the electric motors of motor vehicle starters. Conventionally, such a control device controls the cut-off of the power supply of the electric motor of a starter once the heat engine of the vehicle has been started, or, in the case of failure to start, at the end of a predetermined period of time, so as to avoid any damage to the vehicle engine or an undue drain on the battery.
When the driver of the vehicle, having operated the ignition key, without success, turns it afresh to attempt once again to start the engine, it is desirable that the control device should prevent the starter from being supplied with power if this fresh attempt to start is made too soon. The control device must then include means which enable it to determine, when it is supplied with power again by closing of the contact by the driver, how much time has elapsed between interruption of the starter power supply and the new attempt to start.
The absence of power supplied to the control device during this dead period makes it impossible to provide an internal timer for this purpose in the control device.
Circuits known up to this time for the purpose of measuring the cut-off time of a circuit are generally RC type circuits, the capacitor of which charges when the circuit is supplied with power and discharges when the supply is cut off. When the circuit is once more energized, the measurement of the voltage across the capacitor of the circuit enables a value of the cut-off time to be determined. However, this type of circuit is severely limited as to the lengths of time which it can determine. At best, it only enables determination of time periods of the order of a fraction of a second, because of the leakage currents. In particular, the temperature stability of this type of circuit is not satisfactory, as the leakage currents are very high at high temperatures.
Equally, these RC circuits make it necessary to provide components having high values of capacitance and resistance, the values of which are such as to prevent them from being mounted within the starter casing.